Studies to date have shown that lubricants prepared via the hydro-isomerization of Fischer-Tropsch wax, are equivalent to synthetic lubricants, such as polyalphaolefins (PAO, oligomers of a-decene) in all areas of performance except low temperature performance and base oil oxidation stability. Therefore, a process is needed which is capable of increasing the oxidation stability of hydroisomerized Fischer-Tropsch waxes.
At present, the API "SH" rating is currently employed for passenger car motor oils for use in gasoline engines which represents a significant increase in the service requirements of lubricants from past ratings (e.g., API SF or API SG). Thus, there is a continuing need for lubricants with superior performance characteristics.
One of the performance characteristics which is of greatest significance is the viscosity index (VI). This represents the extent to which the viscosity of a lubricant varies with temperature. Lubricants of high VI change relatively little in viscosity as temperature increases, at least as compared to lubricants of lower VI. Since retention of viscosity at higher temperatures is a desirable characteristic, high viscosity index is desirable. Satisfactory viscosity properties may be conferred either by suitable choice of the lubricant base stock or by the use of VI improvers which are generally high molecular weight polymers.
The extent to which VI properties can be varied by the use of these improvers is, however, limited because not only are large amounts of improver expensive but the improvers are subject to degradation in use so that service life of lubricants containing large amounts of improver may be limited. This implies that improvements in the VI of the base stock are desirable. However, such improvers will have no affect on the oxidation stability of a base stock.
Spectroscopic methods have been used in the art. U.S. Pat. No. 5,475,612 is directed to determining the properties of a liquid hydrocarbon blend from the near IR spectrum of the components of the blend. The method involves determining the absorbance, at a minimum number of frequencies, for the components of arbitrary mixtures. A spectral mixture index is then determined for each component and property. The desired property sought is then calculated by a linear expression.
U.S. Pat. No. 5,419,185 is directed to a method and apparatus for optimizing the extraction of aromatics from waxy distillates and the dewaxing of waxy raffinates in the manufacture of lubricating oils. The method involves irradiating a waxy raffinate, measuring the absorption spectrum and converting the spectrum into a number representative of the VI of the dewaxed oil produced from the waxy raffinate after dewaxing. The aromatics extraction of a waxy distillate which produces the waxy raffinate is then reduced or increased in severity in order to approach a desired VI.
Other articles include "An analysis of petroleum fuels by midband (4000-400/cm) infrared spectroscopy", Foder G. E. et al., SAE International Congress(Detroit 2/28-3/3/94) SAE Meeting Paper N 941019(1994)14P, ISSN 0148-7191 and "Multivariate Calibration in Fourier Transform Infrared Spectrometry for Prediction of Kerosene Properties", Garrigues S. et al., Anal. Chim. Acta., 317(1-3)95-105(1995) Chemical Abstracts ABSTR No. 92150 V124 N. 8 ISSN 0009-2258.
Several articles have been written on thermal diffusion of particular materials such as residual lube stocks (The Composition and Properties of Oil Fractions in Heavy Crudes from the Persian Deposits of Tartarstan, Petrova et al., Russian Academy of Sciences, Kazan Branch, Institute of Organic & Physical Chemistry, Khimiya i Tekhnologiya Topliv I Masel N.5 33-35 (1995) ISSN 0023-1169); MS-8 Oils (Oxidation Tendency of Fractions from the thermal diffusion separation of white oil, Klimov, A.K. et al., Neftepererab. Neftekhim (Moscow) (1979), (1), 23-5 CODEN:NNNSAF; ISSN 0028-1190: Deparaffinated heavy oil, refined neutral II and hydrofined neutral III (Properties of Lubricating oils produced by thermal diffusion, Christakudis et al., Schmierstoffe Schmierungstech. (1969), No. 35, 32-41, CODEN:SSWTBI; and petroleum lube oils (Composition and oxidation of petroleum fractions, Cranton, G. E., Thermochim Acta (1976), 14(1-2), 201-8 CODEN:THACAS.